This invention relates generally to computer-based methods of designing products, and more particularly to a computer-based method of designing an outer air seal for the turbine blades of a gas turbine engine.
An aircraft gas turbine engine generally comprises a compression section, a combustion section and a turbine section. Each section operates on the working fluid in a well-known manner to generate thrust. The compressor and turbine both comprise a plurality of airfoil blades attached to rotating disks to form rotor assemblies. The outermost radial surfaces or tips of the blades in each turbine and compressor rotor are designed to be in close proximity to a corresponding outer air seal. The air seal is part of a shroud assembly attached to the inside of the engine casing and disposed in a radial manner outward from the corresponding turbine or compressor rotor.
The clearance between the blade tips and the outer air seal is sized to reduce the performance penalty that results from air that may leak from the pressure side to the suction side of the blade. As the blade tip clearance increases, an increasing amount of air can leak over the blade tips, causing the compressor and turbine to lose efficiency. Also, the compressor can approach a stall condition, which may be catastrophic for the engine. On the other hand, it is undesirable for the blade tips be in physical contact with the outer air seal, since this could damage the blades and seal. In an effort to reduce these problems, there exist numerous embodiments of active and passive tip clearance controls.
As is well known, a relatively small yet adequate and constant amount of tip clearance must be maintained at every engine operating condition. The tip clearance can vary due to the thermal expansion of the rotors and engine casing as well as the rotational loading applied to the blades and casing. Because of the relatively greater mass of the compressor and turbine rotors compared to the engine casing, the casing experiences larger physical thermal expansion and contraction relative to the rotors, especially during transient engine operating conditions. Maintaining tip clearance is more of a problem with turbine blades than with compressor blades because of the higher turbine operating temperatures. Also, maintaining tip clearance is more of a problem for military engines than with commercial engines. This is due to the more varied and extreme engine transient operating conditions encountered by military aircraft engines.
The outer air seal typically comprises a circular ring made of a plurality of individual segments connected together. Each segment contains a number of primary physical structural features, including means for sealing between adjacent segments (xe2x80x9cinter-segment sealingxe2x80x9d), means for attaching each segment to the engine casing, and means for cooling each segment using, e.g., air bled from the compressor. The inter-segment sealing means typically comprises a ship lap joint that mates with a similar joint on an adjacent segment. The means for attaching each segment to the engine casing typically comprises one or more L-shaped hooks that mate with corresponding grooves in the shroud assembly. The means for cooling each segment typically comprises one or more cooling channels formed within the segment body. Outer air seals typically include other well-known structural features. Examples of outer air seals are given in U.S. Pat. Nos. 5,609,469, 5,374,161 and 5,373,973. All of these patents are assigned to the assignee of the present invention and are incorporated herein by reference.
It is known to design various products using a computer-aided design (xe2x80x9cCADxe2x80x9d) system, a computer-aided manufacturing (xe2x80x9cCAMxe2x80x9d) system, and/or a computer-aided engineering (xe2x80x9cCAExe2x80x9d) system. For sake of convenience, each of these similar types of systems is referred to hereinafter as a CAD system. A CAD system is a computer-based product design system implemented in software executing on a workstation. A CAD system allows the user to develop a product design or definition through development of a corresponding product model. The model is then typically used throughout the product development and manufacturing process. An example is the popular Unigraphics system commercially available from Unigraphics Solutions, Inc. (hereinafter xe2x80x9cUnigraphicsxe2x80x9d).
In addition to CAD systems, there is another type of computer-based product design system which is known as a xe2x80x9cKnowledge-Based Engineeringxe2x80x9d (xe2x80x9cKBExe2x80x9d) system. A KBE system is a software tool that enables an organization to develop product model software, typically object-oriented, that can automate engineering definitions of products. The KBE system product model requires a set of engineering rules related to design and manufacturing, a thorough description of all relevant possible product configurations, and a product definition consisting of geometric and non-geometric parameters which unambiguously define a product. An example is the popular ICAD system commercially available from Knowledge Technologies, Inc. KBE systems are a complement to, rather than a replacement for, CAD systems.
An ICAD-developed program is object-oriented in the sense that the overall product model is decomposed into its constituent components or features whose parameters are individually defined. The ICAD-developed programs harness the knowledge base of an organization""s resident experts in the form of design and manufacturing rules and best practices relating to the product to be designed. An ICAD product model software program facilitates rapid automated engineering product design, thereby allowing high quality products to get to market quicker.
The ICAD system allows the software engineer to develop product model software programs that create parametric, three-dimensional, geometric models of products to be manufactured. The software engineer utilizes a proprietary ICAD object-oriented programming language, which is based on the industry standard LISP language, to develop a product model software program that designs and manipulates desired geometric features of the product model. The product model software program enables the capturing of the engineering expertise of each product development discipline throughout the entire product design process. Included are not only the product geometry but also the product non-geometry, which includes product configuration, development processes, standard engineering methods and manufacturing rules. The resulting model configuration and parameter data, which typically satisfy the model design requirements, comprise the output of the product model software program in ICAD from which the actual product may be manufactured. This output comprises a file containing data (e.g., dimensions) defining the various parameters and configuration features associated with each component or element of the product.
Also, the product model software program typically performs a xe2x80x9cwhat ifxe2x80x9d analysis on the model by allowing the user to change model configuration and/or parameter values and then assess the resulting product design. Other analyses (e.g., a fatigue life analysis) may be run to assess various model features with regard to such functional characteristics as performance, durability and manufacturability. These characteristics generally relate to the manufacturing and operation of a product designed by the product model software program. They are typically defined in terms of boundaries or limits on the various physical parameters of each product feature. The limits have been developed over time based on knowledge accumulated through past design, manufacturing, performance, and durability experience. Essentially, these parameters comprise rules against which the proposed product model design is measured. The rules generally comprise numbers that define physical design limits or constraints for each physical product parameter. Use of these historically developed parameters, analyses, and design procedures in this way is typically referred to as product xe2x80x9crule-based designxe2x80x9d or xe2x80x9cknowledge-based designxe2x80x9d. The rules determine whether the resulting product design will satisfy the component design requirements and is manufacturable or not, given various modem manufacturing processes. The rules for a particular product design are pre-programmed into the product model software program for that specific product.
While the ICAD system provides an excellent tool for developing software product models, it is not a replacement for an organization""s primary CAD system, which maintains the product model definition throughout the entire product design and manufacturing cycle. This is because the ICAD system is a KBE software development tool rather than a CAD system. For example, while the ICAD system can create a geometric model, it cannot easily create drawings based on that model or support other aspects of the design process typically provided by CAD systems. As such, for the product model created in the ICAD system to be useful throughout the entire product development process, the model must be transported into a CAD system for further manipulation.
Another inherent problem with the commercial ICAD system is that the parametric model created by the product model software program cannot be transported as a similar parametric product model into a Unigraphics CAD system. Instead, the parametric model in ICAD must be transported into Unigraphics as a non-parametric model.
Since design and manufacturing technology is always evolving, the product model imported from the ICAD system into Unigraphics will usually be enhanced with new technology design or manufacturing features. Furthermore, since it is difficult to make modifications to a non-parametric model in Unigraphics, revisions to the product model must normally be made in the ICAD system and re-imported into Unigraphics. This causes any additional features previously added in Unigraphics to be lost.
On the other hand, the Unigraphics CAD system has inherent problems in that not all of the parametric models created by Unigraphics are xe2x80x9cstandardizedxe2x80x9d within an organization or industry. Also, parametric models implemented in Unigraphics do not effectively implement a KBE system (similar to the ICAD system) that requires the model configuration and order of Boolean operations to vary according to design requirements. Also, a Unigraphics parametric model cannot be structured to provide parameter relationships that satisfy both design and manufacturing requirements.
As a result, there are instances when a product model developed solely in either the ICAD system or the Unigraphics system will suffice, even with the aforementioned shortcomings. However, there are other instances when a parametric product model developed in the ICAD system is desired to be transported as a corresponding parametric product model and utilized as such in the Unigraphics CAD system.
An object of the present invention is to provide a computer-based method of creating a parametric, three-dimensional, geometric product model of an outer air seal for the turbine blades of a gas turbine engine.
Another object of the present invention is to provide a computer-based method of creating a parametric product model in a KBE system that can be re-created as a similar parametric product model in a CAD system.
According to a first aspect of the present invention, a method of designing an outer air seal for the turbine blades of a gas turbine engine utilizes a knowledge-based product model software program for generating a parametric, three-dimensional, geometric model of the air seal. The product model software program is embodied in a KBE system. The resulting product model may implement many different configurations of the structural features of the air seal. The product model is created by the program through user selection of various structural feature options available for the air seal. The product model software program uses its internal knowledge base of configuration dependent parameter relationships and constraints or rules to create a parametric, three-dimensional, geometric model of the air seal. The internal knowledge base is pre-programmed into the product model software program. Various types of durability, performance and manufacturability analyses may then be run on features of the product model to validate the model. The model may be changed, if necessary, as a result of the analyses. The air seal model output from the product model software program is in a file format that defines the configurations and dimensions of the geometry of the air seal. Other software programs may then use this product model output file in various ways. The product model software program also creates a design report and a non-parametric geometry model.
According to a second aspect of the present invention, the geometric specific information regarding the product model of the outer air seal is output in file format from the product model software program into a computer program. The program generates a parametric, three-dimensional, geometric model of the outer air seal that can be used by a CAD system. The program generates the model using a library of various standardized physical features of the outer air seal. This library of features resides in the workstation file system. Once the parametric model is generated, the CAD system operates on the model, allowing the user to manipulate the model in various ways not available to the user in the product model software program implemented in the KBE system. The CAD system allows the user to rapidly make changes to the parametric model without having to return to the product model software program.
The above and other objects and advantages of the present invention will become more readily apparent when the following description of a best mode embodiment of the present invention is read in conjunction with the accompanying drawings.